Closures for pressure vessels and pipes typically comprise a round door adapted to fit an aperture in the body of the vessel, pipe, or hub extending therefrom. The aperture has an inner circumferential groove for supporting a portion of a locking member. The doors have locking member(s), or portions thereof, proximate a circumferential portion of an outer surface of the door wherein the locking member is engaged to cooperate with the groove about the aperture after the door is closed and seated in the aperture. The locking members of the prior art are typically comprised of a plurality of arms or blocks slidably mounted adjacent the outer periphery of the door and movable from an unlocked position to a locked position. In an unlocked position, the locking member(s) is within the periphery of the outer surface of the door. In a locked position, a part of the locking member(s) protrudes beyond the periphery for engagement with the circumferential groove in the aperture.
Known configurations of locking members include arms or blocks that rotate or extend outwardly, arcuate or “C” shaped rings, slotted as well as segmented, that expand to a locking configuration and contract to an unlocking configuration. Locking member(s) of the prior art have load bearing surfaces that engage an external surface of the annular groove in the hub or vessel and a load bearing surface that engages a portion of the external surface of the door. When the internal surface of the door is subjected to pressure, the resultant force is transferred from the external surface of the door to the external surface of the annular groove about the aperture through the locking member(s) wherein an internal load bearing surface cooperates with the external surface of the door and an external load bearing surface cooperates with the external load bearing surface of the annular groove of the hub.